Abstract
<jats:p>Abstract: Cryptic species complexes, characterized by high morphological similarity and substantial niche overlap, remain poorly understood with respect to how biotic interactions shape interspecific niche differentiation. We exposed four formally delimited species of the Brachionus calyciflorus cryptic species complex to Asplanchna girodi kairomones and A. girodi live individual to investigating morphological and life-history inducible defense responses and quantifying their survival benefits. In the control treatment, the four species differed significantly in body size and anterior spine length. Differences in life-history traits were primarily reflected in the allocation of time across developmental and reproductive stages, as well as in total offspring number. Exposure to kairomones induced species-specific morphological responses: posterolateral spine elongation emerged as a convergent, universal defense across all species, whereas body size exhibited clear interspecific divergence. B. elevatus uniquely reduced body size, while the other three species increased it. Life-history responses were less obvious, species-specific, and served as a compensatory defensive function. More importantly, the survival benefit under direct predation matched the magnitude of morphological plasticity, reconfirming that defensive trait divergence carries functional ecological consequences. The findings demonstrate that convergence in a core morphological defense, coupled with divergence in life history traits, may contribute to defensive niche partitioning among sympatric cryptic species under shared predation pressure. Our results advance the understanding of predation as a biotic driver of niche differentiation in cryptic species complexes, a phenomenon with broad relevance for the maintenance of biodiversity in freshwater ecosystems.</jats:p>